This invention relates to phase shift fabrication methods, to methods of determining processing alignment in the forming of phase shift regions in the fabrication of a phase shift mask, to methods of determining photoresist pattern alignment in the forming of phase shift regions in the fabrication of a phase shift mask, and to phase shift masks.
In semiconductor manufacturing, photolithography is typically used in the formation of integrated circuits on a semiconductor wafer. During a lithographic process, a form of radiant energy such as ultraviolet light is passed through a mask/reticle and onto the semiconductor wafer. The mask contains light restricting regions (for example totally opaque or attenuated/half-tone) and light transmissive regions (for example totally transparent) formed in a predetermined pattern. A grating pattern, for example, may be used to define parallel-spaced conductive lines on a semiconductor wafer. The wafer is provided with a layer of photosensitive resist material commonly referred to as photoresist. Ultraviolet light passed through the mask onto the layer of photoresist transfers the mask pattern therein. The resist is then developed to remove either the exposed portions of resist for a positive resist or the unexposed portions of the resist for a negative resist. The remaining patterned resist can then be used as a mask on the wafer during a subsequent semiconductor fabrication step, such as ion implantation or etching relative to layers on the wafer beneath the resist.
Advances in semiconductor integrated circuit performance have typically been accompanied by a simultaneous decrease in integrated circuit device dimensions and in the dimensions of conductor elements which connect those integrated circuit devices. The wavelength of coherent light employed in photolithographic processes by which integrated circuit devices and conductors are formed has typically desirably been smaller than the minimum dimensions within the reticle or mask through which those integrated circuit devices and elements are printed. At some point, the dimension of the smallest feature opening within the reticle approaches the wavelength of coherent light to be employed. Unfortunately, the resolution, exposure latitude and depth of focus in using such reticles and light decreases due to aberrational effects of coherent light passing through openings of width similar to the wavelength of the coherent light. Accordingly as semiconductor technology has advanced, there has traditionally been a corresponding decrease in wavelength of light employed in printing the features of circuitry.
One approach for providing high resolution printed integrated circuit devices of dimensions similar to the wavelength of coherent light utilized employs phase shift masks or reticles. In comparison with conventional reticles, phase shift masks typically incorporate thicker or thinner transparent regions within the conventional chrome metal-on-glass reticle construction. These shifter regions are designed to produce a thickness related to the wavelength of coherent light passing through the phase shift mask. Specifically, coherent light rays passing through the transparent substrate and the shifter regions have different optical path lengths, and thus emerge from those surfaces with different phases. By providing transparent shifter regions to occupy alternating light transmitting regions of the patterned metal layer of a conventional phase shift mask of the Levenson type, adjacent bright areas are formed preferably 180xc2x0 out-of-phase with one another. The interference effects of the coherent light rays of different phase provided by a phase shift mask form a higher resolution image when projected onto a semiconductor substrate, with accordingly a greater depth of focus and greater exposure latitude.
Another type of phase shift mask is referred to as an attenuated or half-tone phase shift mask. The attenuated phase shift mask has formed upon a transparent substrate a patterned semitransparent shifter layer. Such is typically formed of an oxidized metal layer which provides a 180xc2x0 phase shift to the coherent light rays utilized with the mask, and produces a light transmissivity typically in a range of from 4% to 30%.
Certain equipment referred to as aerial image measurement equipment has been developed to determine the degree of phase shift obtained in the fabrication of phase shift regions. A goal or desire in the fabrication of such regions is to achieve exactly 180xc2x0 out-of-phase from an adjacent reference region. In the fabrication however, the phase shift obtained might be some amount xe2x80x9coffxe2x80x9d of 180xc2x0 depending upon the typical thickness change phase shift region which is obtained. One example aerial image measurement equipment is the Microlithography Simulation Microscope MSM 100(trademark) available from Carl Zeiss, Inc. of Thornwood, N.Y.
The invention comprises phase shift fabrication methods, methods of determining processing alignment in the forming of phase shift regions in the fabrication of a phase shift mask, methods of determining photoresist pattern alignment in the forming of phase shift regions in the fabrication of a phase shift mask, and phase shift masks. In but one implementation in the fabrication of a phase shift mask, both process alignment in the formation of a phase shift alignment region and degree of phase shift of the phase shift alignment region is determined at least in part by using aerial image measurement equipment. In one implementation in the fabrication of a phase shift mask, aerial image measurement equipment is used to both determine phase shift of a phase shift alignment region at least in part by capturing a series of aerial images as a function of focus and to determine process alignment in the formation of the phase shift alignment region at least in part by measuring distance between spaced low intensity locations defined by an edge of the phase shift alignment region and an adjacent alignment feature edge.
In one implementation in the fabrication of a phase shift mask, process alignment in the formation of a phase shift alignment region is determined at least in part by using aerial image measurement equipment to determine photoresist patterning alignment prior to etching material to form said phase shift alignment region. In one implementation in the fabrication of a phase shift mask, aerial image measurement equipment is used to determine photoresist patterning alignment for formation of a phase shift alignment region at least in part by measuring distance between spaced intensity change locations defined by an alignment feature edge beneath the photoresist and an edge of the photoresist.